1. Field of the Invention
The present invention relates to a high-density magneto-optical recording medium and a reproducing method for information recorded on the medium.
2. Description of the Related Art
A magneto-optical disk is known as a high-density recording medium, and an increase in information quantity gives rise to a desire for higher densities of the medium. While the higher densities may be realized by reducing the space of recorded marks, the recording and reproducing of the marks are limited by the size of a light beam (beam spot) on the medium. When the presence of only one recorded mark in the beam spot is set, an output waveform corresponding to xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d may be observed as a reproduction signal according to whether the recorded mark is present or absent in the beam spot.
However, when the presence of plural recorded marks in the beam spot is set by reducing the space of the recorded marks, no change in reproduction output occurs regardless of movement of the beam spot on the medium. Accordingly, the output waveform becomes linear and the presence or absence of the recorded mark in the beam spot cannot be identified. To reproduce such small recorded marks having a period smaller than the size of the beam spot, it is sufficient to reduce the beam spot to a small size. However, since the size of the beam spot is limited by the wavelength xcex of a light source and the numerical aperture NA of an objective lens, the beam spot cannot be sufficiently reduced to a small size.
There has recently been proposed a reproducing method using magnetically induced super resolution such that a recorded mark smaller in size than the beam spot can be reproduced by the use of an existing optical system. According to this method, the resolution of reproduction is improved by masking other marks during reproduction of one mark in the beam spot. Accordingly, a super resolution disk medium is required to have at least a mask layer or a reproducing layer for masking other marks so that only one mark may be reproduced during signal reproduction, in addition to a recording layer for recording marks. A magneto-optical recording medium using a perpendicular magnetization film as the reproducing layer is proposed in Japanese Patent Laid-open No. 3-88156. In the prior art described in this publication, however, an initializing magnetic field of about several kOe is required to initialize the reproducing layer. Accordingly, a recording apparatus cannot be made compact.
On the other hand, a magneto-optical recording medium using a magnetic film as the recording layer is proposed in Japanese Patent Laid-open Nos. 5-81717 and 5-342670. This magnetic film has an easy direction of magnetization in a plane at room temperature and has an easy direction of magnetization perpendicular to a film surface at a given temperature or higher. The principle of reproduction in this prior art will now be described in brief with reference to FIGS. 31A, 31B, and 31C. As shown in FIG. 31C, a magneto-optical disk 2 is formed by laminating a magnetic reproducing layer 6 and a magnetic recording layer 8 on a transparent substrate 4. The magnetic reproducing layer 6 has an easy direction of magnetization in a plane at room temperature. However, when the medium is heated to a given temperature or higher by applying a reproducing power, the easy direction of magnetization is changed to a perpendicular direction. The magnetic recording layer 8 is a perpendicular magnetization film. Reference numeral 10 denotes a light beam.
The intensity distribution of the light beam is a Gaussian distribution as shown in FIG. 31A. Accordingly, when the disk is at rest, the temperature distribution on the disk is also a similar distribution such that the central portion is higher in temperature than the peripheral portion. In operation, however, the disk 2 is rotated in the direction of arrow R shown in FIG. 31C during reproduction. Accordingly, the temperature distribution on the disk in rotation becomes a distribution as shown in FIG. 31B so that a high-temperature area in the beam spot is shifted to the forward direction of rotation of the disk. Owing to such a temperature distribution during reproduction, the easy direction of magnetization of the magnetic reproducing layer 6 becomes an in-plane direction in a low-temperature area in the beam spot. Therefore, the Kerr rotation angle of reflected light becomes almost zero in the low-temperature area. In the high-temperature area, the easy direction of magnetization of the magnetic reproducing layer 6 is changed from an in-plane direction to an perpendicular direction.
The perpendicular magnetization of the magnetic reproducing layer 6 at this time is bonded to the magnetization of the magnetic recording layer 8 by an exchange force, and the direction of magnetization of the reproducing layer 6 is made identical with the direction of magnetization of the recording layer 8, thereby allowing the magnetization recorded in the recording layer 8 to be transferred to the reproducing layer 6. The area size of such transfer can be changed by varying a reproducing laser beam power. In this manner, the size of the masking reproducing layer is controlled so as to allow the reproduction of only one recorded mark, thereby obtaining the same effect as that in the case of substantially reducing the area of the beam spot.
As mentioned above, the intensity distribution of the laser beam 10 directed onto the disk 2 is a Gaussian distribution, and the disk 2 is rotated in the direction of arrow R. As a result, a low-temperature area 10a and a high-temperature area 10b are formed on the reproducing layer 6 (see FIG. 32). The high-temperature area 10b is shifted to the forward direction of rotation of the disk 2 with respect to the laser beam 10. In the prior art disclosed in Japanese Patent Laid-open No. 5-81717, however, the in-plane magnetization of the reproducing layer 6 in the low-temperature area 10a in the beam spot is bonded to the perpendicular magnetization of the recording layer 8, causing inclination of the in-plane magnetization to generate a perpendicular component as shown in FIG. 32.
As a result, the masking effect is reduced and a mark recorded on the recording layer adjacent to a mark to be reproduced cannot be perfectly masked. Accordingly, the magnetization of the recording layer in the low-temperature area is also transferred to the reproducing layer, so that individual marks cannot be identified because of interference causing a reduction in reproduction output.
Japanese Patent Laid-open No. 5-342670 mentioned above discloses a magneto-optical recording medium having a magnetic intermediate layer interposed between a magnetic reproducing layer and a magnetic recording layer. The magnetic intermediate layer is provided to prevent the possibility that when the exchange bonding force between the recording layer and the reproducing layer is too strong, the magnetization direction of the reproducing layer becomes perpendicular in an area where the laser beam is not directed, thereby reducing the masking effect of the reproducing layer. The magnetic intermediate layer described in this publication is considered from its composition to have a Curie point lower than a temperature of the medium to be heated by the reproducing laser beam. While the operation of the magnetic intermediate layer is not described in detail in this publication, it may be considered as follows:
When the magnetic intermediate layer is heated to temperatures higher than its Curie temperature, the magnetization of the intermediate layer disappears. At this time, in the low-temperature area in the beam spot, a stable in-plane mask is formed in the reproducing layer, while in the high-temperature area, the magnetization of the recording layer is transferred to the reproducing layer by a magnetostatic bond. Accordingly, information recorded on the medium in the high-temperature area can be read out. However, the transfer of the magnetization by a magnetostatic bonding force is weaker than the transfer of the magnetization by an exchange bonding force. Thus, the medium having the magnetic intermediate layer described in this publication is not satisfactory in transfer characteristic of the magnetization in the high-temperature area in the beam spot. In addition, the magnetostatic bonding force between the recording layer and the reproducing layer is absorbed by the magnetic intermediate layer, thus further hindering the transfer characteristic of the magnetization.
It is therefore an object of the present invention to provide a magneto-optical recording medium which can perfectly mask a mark adjacent to a mark to be reproduced to thereby improve a reproduction output.
It is another object of the present invention to provide a magneto-optical recording medium which can prevent the crosstalk between a track to be reproduced and a track adjacent to this track to thereby realize the improvement in the reproduction output.
According to a first aspect of the present invention, there is provided a magneto-optical recording medium comprising a transparent substrate; a magnetic reproducing layer laminated on said transparent substrate, said reproducing layer having an easy direction of magnetization in a plane at room temperature and having an easy direction of magnetization perpendicular to a film surface at a given temperature or higher; a nonmagnetic intermediate layer laminated on said reproducing layer; and a magnetic recording layer laminated on said nonmagnetic intermediate layer, said recording layer having an easy direction of magnetization perpendicular to a film surface; wherein said nonmagnetic intermediate layer is thin enough to allow magnetostatic bond between said recording layer and said reproducing layer.
Preferably, the nonmagnetic intermediate layer has a thickness ranging from 1 nm to 10 nm. The nonmagnetic intermediate layer is formed from a substance selected from the group consisting of Al, Si, Ti, oxides, and nitrides thereof.
According to the first aspect, the nonmagnetic intermediate layer is interposed between the magnetic recording layer and the magnetic reproducing layer, so that the exchange bonding force between the two magnetic layers can be perfectly cut off. Owing to the sufficiently small thickness of the nonmagnetic intermediate layer, when the reproducing power is applied to the medium to heat the reproducing layer to the given temperature or higher, the magnetization of the recording layer is transferred to the reproducing layer by the magnetostatic interaction, thereby reproducing the information recorded on the recording layer. The exchange interaction between the recording layer and the reproducing layer is cut off by the nonmagnetic intermediate layer, thereby preventing the inclination of magnetization of the reproducing layer from the in-plane direction due to the exchange interaction in a low-temperature area in a beam spot, with the result that the reproduction output can be improved.
According to a second aspect of the present invention, there is provided a magneto-optical recording medium comprising a transparent substrate; a magnetic reproducing layer laminated on said transparent substrate, said reproducing layer having an easy direction of magnetization in a plane at room temperature and having an easy direction of magnetization perpendicular to a film surface at a given temperature or higher; a magnetic intermediate layer laminated on said reproducing layer, said magnetic intermediate layer having an easy direction of magnetization in a plane; and a magnetic recording layer laminated on said magnetic intermediate layer, said recording layer having an easy direction of magnetization perpendicular to a film surface.
Preferably, the magnetic intermediate layer is formed from a light rare earth-transition metal amorphous alloy film represented by RXFeXCo1xe2x88x92Xxe2x88x92Y (R=Nd, Sm), where 0 less than X less than 0.5 and 0xe2x89xa6Y less than 0.5.
According to the second aspect, the magnetic intermediate layer having an easy direction of magnetization always in a plane is interposed between the magnetic recording layer and the magnetic reproducing layer. Accordingly, the in-plane magnetization of the magnetic intermediate layer is stable in a low-temperature area in a beam spot in applying a reproducing power to the medium, so that there does not occur the inclination of magnetization of the magnetic intermediate layer due to the perpendicular magnetization of the recording layer. In a high-temperature area in the beam spot, the easy direction of magnetization of the reproducing layer is perpendicular. At this time, the perpendicular direction of magnetization of the magnetic intermediate layer becomes an easy direction of magnetization because of the perpendicular magnetization of both the recording layer and the reproducing layer, and is made identical with the direction of magnetization of the recording layer by the exchange bond to the magnetization of the recording layer. Furthermore, the magnetization of the reproducing layer is exchange-bonded to the magnetization of the magnetic intermediate layer, and the direction of magnetization of the reproducing layer therefore becomes identical with the direction of magnetization of the intermediate layer. As a result, the direction of magnetization of the recording layer is transferred to the reproducing layer.
According to a third aspect of the present invention, there is provided a magneto-optical recording medium comprising a transparent substrate; a magnetic opening portion control layer laminated on said transparent substrate, said control layer having an easy direction of magnetization in a plane and having a transmittance of 60% or more; a magnetic reproducing layer laminated on said control layer, said reproducing layer having an easy direction of magnetization in a plane at room temperature and having an easy direction of magnetization perpendicular to a film surface at a given temperature or higher; and a magnetic recording layer laminated on said reproducing layer, said recording layer having an easy direction of magnetization perpendicular to a film surface.
Preferably, a Curie temperature Tc1 of said control layer, a Curie temperature Tc2 of said reproducing layer, a Curie temperature Tc3 of said recording layer, a room temperature Troom, and a temperature Tread of said reproducing layer in applying a reproducing power thereto are related to satisfy Tc2 greater than Tc3 greater than Tc1 greater than Troom, and Tread greater than Tc1 at an opening portion of said control layer.
According to the third aspect, the control layer having a transmittance of 60% or more is provided. As the low-temperature area in the reproducing beam spot is subjected to temperatures lower than the Curie temperature of the control layer, the in-plane magnetization remains in the control layer. As a result, the magnetization of the reproducing layer follows the magnetization of the control layer owing to the exchange bonding force to become the in-plane magnetization. Accordingly, the control layer serves as a mask in the low-temperature area, thereby making impossible the reproduction of a mark in the reproducing layer. In the high-temperature area in the reproducing beam spot, the control layer is heated to temperatures higher than the Curie temperature, causing disappearance of the magnetization of the control layer. As a result, the bond in magnetization between the reproducing layer and the control layer is cut to bring the magnetization of the reproducing layer into perfectly perpendicular magnetization, thereby allowing the reproduction of the mark in the reproducing layer through the control layer.
According to a fourth aspect of the present invention, there is provided a magneto-optical recording medium comprising a transparent substrate; a magnetic reproduction assisting layer laminated on said transparent substrate, said assisting layer having an easy direction of magnetization perpendicular to a film surface; a magnetic reproducing layer laminated on said assisting layer, said reproducing layer having an easy direction of magnetization in a plane at room temperature; and a magnetic recording layer laminated on said reproducing layer, said recording layer having an easy direction of magnetization perpendicular to a film surface; wherein a Curie temperature Tc1 of said assisting layer, a Curie temperature Tc2 of said reproducing layer, and a Curie temperature Tc3 of said recording layer are related to satisfy Tc3 less than Tc1 and Tc3 less than Tc2; and a coercive force Hc1 of said assisting layer and a coercive force Hc3 of said recording layer are related to satisfy Hc3 greater than Hc1.
According to the fourth aspect, the direction of magnetization of the reproducing layer is an in-plane direction in the low-temperature area in the beam spot. Accordingly, the direction of magnetization of the assisting layer is made identical with the direction of a bias magnetic field to form a perpendicular mask. In the high-temperature area in the beam spot, the magnetization of the reproducing layer is exchange-bonded to the magnetization of the recording layer, and the magnetization of the reproducing layer is also exchange-boned to the magnetization of the assisting layer. Accordingly, the direction of magnetization of the recording layer is transferred to the assisting layer, thereby allowing reading of information recorded in the recording layer.
Accordingly, when a magneto-optical output is differentially detected, the low-temperature area in the beam spot acts as a mask, so that a magneto-optical signal in the low-temperature area is not read out, but a magneto-optical signal only in the high-temperature area is read out. Therefore, a mark having a size less than the diffraction limit of a reproducing laser wavelength can be read out.
According to another aspect of the present invention, there is provided a reproducing method for information recorded on a magneto-optical recording medium comprising a transparent substrate; a magnetic reproducing layer laminated on said transparent substrate, said reproducing layer having an easy direction of magnetization in a plane at room temperature and having an easy direction of magnetization perpendicular to a film surface at a given temperature or higher; a nonmagnetic intermediate layer laminated on said reproducing layer; and a magnetic recording layer laminated on said nonmagnetic intermediate layer, said recording layer having an easy direction of magnetization perpendicular to a film surface; wherein said nonmagnetic intermediate layer is thin enough to allow magnetostatic bond between said recording layer and said reproducing layer; said reproducing method comprising the steps of directing a laser beam onto said recording medium as applying a bias magnetic field to heat said recording medium to temperatures lower than the Curie temperature of said recording layer; and forming a temperature distribution in a beam spot, said temperature distribution comprising a low-temperature area where the direction of magnetization of said reproducing layer is an in-plane direction, an intermediate-temperature area where Hrxe2x89xa6Hs+Hc is satisfied and magnetization of said recording layer is transferred to said reproducing layer by magnetostatic bond, and a high-temperature area where Hr greater than Hs+He is satisfied and the direction of magnetization of said reproducing layer is identical with the direction of said bias magnetic field; where Hr represents a strength of said bias magnetic field, Hs represents a magnetostatic bonding force between said reproducing layer and said recording layer, and Hc represents a coercive force of said reproducing layer.
Preferably, the reproducing layer and the recording layer are formed from a rare earth-transition metal amorphous alloy film. When the recording layer is formed from a rare earth rich rare earth-transition metal amorphous alloy film, there is formed in the beam spot a temperature distribution comprising a low-temperature area where the direction of magnetization of the reproducing layer is made identical with the direction of the bias magnetic field, an intermediate-temperature area where the magnetization of the recording layer is transferred to the reproducing layer, and a high-temperature area where the direction of magnetization of the reproducing layer is made identical with the direction of the bias magnetic field.
According to still another aspect of the present invention, there is provided a reproducing method for information recorded on a magneto-optical recording medium comprising a transparent substrate; a magnetic reproducing layer laminated on said transparent substrate, said reproducing layer having an easy direction of magnetization in a plane at room temperature and having an easy direction of magnetization perpendicular to a film surface at a given temperature or higher; a nonmagnetic intermediate layer laminated on said reproducing layer; and a magnetic recording layer laminated on said nonmagnetic intermediate layer, said recording layer having an easy direction of magnetization perpendicular to a film surface; wherein said nonmagnetic intermediate layer is thin enough to allow magnetostatic bond between said recording layer and said reproducing layer; said reproducing method comprising the steps of directing a laser beam onto said recording medium to heat said recording medium to temperatures lower than the Curie temperature of said recording layer; and forming a temperature distribution in a beam spot, said temperature distribution comprising a low-temperature area where the direction of magnetization of said reproducing layer is an in-plane direction, an intermediate-temperature area where magnetization of said recording layer is transferred to said reproducing layer by magnetostatic bond, and a high-temperature area where said magnetization transferred to said reproducing layer is spontaneously extinguished by high temperatures to make the direction of magnetization of said reproducing layer identical with the direction of magnetization to be erased.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.